Ammonium nitrate composition



United ates 3,148,946 AMMGNIUM NITRATE CQMPOSITIDN Edward J. Grifith, Manchester, Mm, assignor to Monsanto Company, a corporation of Delaware No Drawing. Filed Sept. 28, N60, Ser. No. 58,881 2 Ciaims. (Cl. 231tt3) This invention relates to improved ammonium nitrate compositions. More specifically, the invention relates to solid ammonium nitrate compositions having improved resistance to crystalline phase change.

It is well known that ammonium nitrate and most compositions containing more than a very small amount of ammonium nitrate are subject to one very significant shortcoming which tends to limit their general usage. That is, that solid ammonium nitrate undergoes changes in volume and/ or shape when its temperature is varied through the region of about 30-40 C. These changes in volume and shape are attributed to a crystalline phase change through which ammonium nitrate normally passes under these conditions. If solid ammonium nitrate is exposed to temperatures which are cycled through the region of about'30-40 C., the individual particles often crumble, breaking down to a fine powder which cakes readily, swells excessively, absorbs moisture rapidly, etc., and generally results in a product which is very difiicult to handle and use. monium nitrate changes in volume as much as 50-60 percent or more when its temperature is raised and lowered (cycled) several times through the temperature region of about 30-40 C.

In addition, for some potential uses in which it would be desirable to use ammonium nitrate in a solid molded form (for example, as rocket propellents, shaped explosive charges, etc.), the well known dimensional instability of ammonium nitrate has, to date, almost precluded its consideration.

Various additives and techniques have been proposed for counteracting the above-described dimensional instability of ammonium nitrate. However, to date, only a very few have been commercially successful.

It is a primary object of this invention to provide ammonium nitrate compositions having significantly improved resistance to physical deformation due to temperature variations through the range of Sit-40 C.

The above object, as Well as other objects of the invention are achieved by incorporating into the ammonium nitrate a minor proportion by weight of ferrous sulfate.

Reasons for the observed improvement in dimension-a1 stability of ammonium nitrate compositions prepared according to the present inventoin are not known with certainty, but it is believed that this particular ferrous salt fits well into the ammonium nitrate crystalline structure, possibly forming a double salt with the ammonium nitrate, and thereby interfering with or inhibiting certain of the usual properties of ammonium nitrate. In view of the well known fact that most ferrous salts are almost immediately converted into ferric salts when they are incorporated into an oxidizing medium such as ammonium nitrate, it is surprising that ferrous sulfate, specifically, is not so oxidized. It is particularly surprising too, that ferrous sulfate actually improves the dimensional stability of ammonium nitrate, since most other iron salts are known to be inefiective for this purpose. Although the actual amount of the ferrous sulfate which is incorporated into the ammonium nitrate Willdepend to some extent upon the actual amount of moisture which will be permitted to remain in the final solid ammonium nitrate composition, generally, only about 0.05 weight percent (based on the ammonium nitrate) of the ferrous sulfate is sufificient to effect a'significant improvement in the dimensional stability of the ammonium nitrate. When Normally, for example, solid am ddddfidfi Patented Sept. 15, 1964,

ice

based on the weight of the ammonium nitrate, of ferrous sulfate. Apparently the upper limit of improvement in the dimensional stability and hygroscopicity of ammonium nitrate which can be observed withadditional quantities of the ferrous sulfate in the ammonium nitrate is about 10 weight percent. Therefore, in the practice'of this in-.

vention, more than about 10 weight percent of ferrous sulfate is not normally incorporated into the ammonium nitrate.

The ferrous sulfate is introduced into the ammonium nitratein any manner which will enable it to be distributed evenly through the ammonium nitrate composition. For example, solidified ammon-ium nitrate can be mixed with ferrous sulfate, and the resulting mixture thereafter heated to a molten state and prilled. It is preferred, however, that the ferrous sulfate either be incorporated into the ammonium nitrate while the ammonium nitrate is in the form of a concentrated liquor or melt (generally at a tem: perature between about 110? C. and about 200 C.), prior to the cooling or prilling step of the conventional ammonium nitrate preparation procedure, or be inter mixed with one or more of the raw materials which are to be used in the manufacture of the ammonium nitrate composition, such as, for example, nitric acid, which use is illustrated in the following simple, yet effective procedure.

In the following specific embodiments of the abovedescribed process for incorporating ferrous sulfate into the ammonium nitrate, all parts are by weight unless otherwise noted.

One hundred parts of anhydrous ammonia are reacted with 675 parts of 55% by weight aqueous nitric acid to yield 555 parts of aqueous ammonium nitrate solution. Into 40 parts of the resulting solution in a separate container are dissolved 9.3 parts of ferrous sulfate. This latter solution is filtered to remove any insoluble impuri ties and is then returned to the aforementioned 85% ammonium nitrate solution. This solution is then concentrated to a solution by evaporating water therefrom at about C. and about one-half atmosphere, and thereafter sprayed and cooled in a conventional prilling tower. The resulting prills are then kiln-dried to yield an ammonium nitrate composition containing 2.0% ferrous sulfate.

Another way in which ferrous sulfate can be combined With ammonium nitrate is illustrated below: 7 To 200 parts of ammonium nitrate liquor, containing 85% by weight of ammonium nitrate, and at a tempera ture of 140 C. is added 1.7 parts'of ferrous sulfate. The resulting mixture is blended for several minutes, and then further concentrated by additional heating to 96% by weight solids at a temperature of C.-and under onehalf atmosphere of'pressure. The 96% solids liquor is then sprayed and cooled in a conventional prilling tower. The resulting prills are subsequentlyconventionally kilndried :to yield an ammonium nitrate composition containing ammonium nitrate and 1.0% by weight ferrous sulfate, based on the weight of the ammonium nitrate, and less than 0.1% by weight ofwater. This product is then stored until used under essentially anhydrous conditions.

The surprising benefit-s which result from the practice of this invention are illustrated below.

Forty grams of the ammonium nitrate composition prepared above (with 1.0% ferrous sulfate) are crumbled into pieces small enough to pass through a US. Standard 40-mesh sieve. These pieces are then poured into a inch diameter by 8-inch long test-tube. This amount of the composition fills approximately the lower three inches of the test-tube. The test-tube containing the ammonium nitrate composition is then cycled 23 times through the temperature range of about 3040 C. as follows.

First the sample is warmed to and held at 40 C. for 2 hours. Then it is cooled to and held at 25 C. for 1 hour. This procedure represents one complete cycle.

Observations of the dimensional stability of the ammonium nitrate composition comprises measuring the increase in volume (increase in height in the test-tube occupied by the composition) of the ammonium nitrate after 23 complete cycles. Samples of ammonium nitrate which do not increase in volume during this cycling-temperature treatment are more dimensionally stable than samples which noticeably do increase in volume during the same treatment. After being subjected to 23 cycles of tern: perature, the sample of ammonium nitrate prepared with 1.0% ferrous sulfate. shows no expansion in volume, while a sample of substantially pure commercial ammonium nitrate, which contains 0.1% water and none of the ferrous sulfate, but which is otherwise treated similarly, increases in volume 'by 50%.

It should be recognized that the practice of the present invention does not preclude the presence or addition of materials other than the ferrous sulfate described herein. For example, many of the material's conventionally added to or coated upon ammonium nitrate to reduce hygro scopicity or to enhance the flowability of the granular or prilled solids may also :be used in conjunction with the compositions of this invention. Typical examples of such materials are talc, diatomaceou-s earth, tricalcium phosphate, organic hydrocarbon or resinous coating agents, etc. Liksewise other inorganic chemicals can be added to the present ammonium nitrate compositions, as by adding such agents to the molten compositions before solidification thereof. For example, potassium nitrate has in the past been added to ammonium nitrate in order to lower the temperature at which the phase transition takes place. Likewise other plant nutrient materials such as phosphates and minor essential elements such as copper, manganese, etc., have been added to ammonium nitrate in order to provide various multicomponent fertilizer compositions. Such materials can also be incorporated into or utilized with the products of the present invention without adversely affecting these products.

.What is claimed is:

1. Prills of ammonium nitrate having between about 0.05 and about 10 weight percent of ferrous sulfate substantially uniformly distributed therethrough.

2. Granular ammonium nitrate having dispersed there through between about 0.05 and about 10 Weight percent of ferrous sulfate. I

References Cited in the file of this patent UNITED STATES PATENTS 1,966,947 Eyer et a1. July 17, 1934 2,739,036 Kamenjar et al Mar. 20, 19.56 2,795,495 Schmatlock et a1 June 11, 1957 

1. PRILLS OF AMMONIUM NITRATE HAVING BETWEEN ABOUT 0.05 AND ABOUT 10 WEIGHT PERCENT OF FERROUS SULFATE SUBSTANTIALLY UNIFORMLY DISTRIBUTED THERETHROUGH. 